Metallurgical Terms
V
V
Chemical symbol for vanadium
Vacancy
An empty atomic site within the crystal lattice. Vacancies can move about
within the crystal under the application of stress.
Vacuum Degassing
By casting steel in a vacuum it is possible to reduce considerably the gas
content, particularly hydrogen. Other advantages are also obtainable, such as a
reduction in non-metallic inclusions. In stream degassing, molten metal is
poured into an ingot mould or a ladle previously placed within an evacuated
chamber. The molten stream is broken up into droplets by gas evolution which
takes place very rapidly. In ladle degassing, the ladle of molten metal is
placed within a chamber, which is then evacuated; an inert gas such as argon or
helium may be used to stir the molten metal and thus assist hydrogen removal.
Other methods of ladle degassing can be used in which the liquid steel is
siphoned into an evacuated chamber and returned to the ladle. The ladle of
degassed metal can be subsequently poured into moulds in air in the normal
manner.
Vacuum Melting
Two methods are used for steel. Vacuum induction melting uses a high
frequency induction furnace contained in a chamber which can be evacuated.
Arrangements are made for adding alloys and for casting into moulds without
breaking the vacuum. The other method, consumable electrode re-melting, uses an
ingot or billet as an electrode from which an arc is struck, melting the steel
into a water-cooled copper crucible and building up an ingot which can be
subsequently forged or rolled in the normal manner. No alloy additions can be
made during this process, so the electrode must approximate to the required
composition. With both methods there is a reduction in the content of gas and of
some other elements, such as manganese, which are volatile at the low pressures
and high temperatures operating in the vacuum chamber. Improved cleanness is
also obtained, although in the vacuum induction process the metal may be
contaminated to some extent with oxides from the refractory lining of the
furnace. In the consumable are process, since only a small pool of metal is
molten at any one time, segregation is usually less marked than in the product
of other melting processes and the continuous freezing from the bottom gives a
very sound ingot.
Vickers Hardness Test
A diamond pyramid of 136˚ angle is pressed into the polished surface of the
specimen and the diagonals of the impression are measured with a microscope
fitted with a micrometer eyepiece. The rate of application and duration of
loading are automatically controlled and the load can be varied from 1 to 120
kg. The hardness number is obtained by dividing the load by the surface area of
the indentation.
Vitreous Fracture
(See Fracture)